An underwater detector detects a state underwater by driving a transducer to transmit an ultrasonic signal and thereafter, receives an echo signal reflected by fish and a target such as the bottom of the sea. At this time, the underwater detector is required to have a capability of suppressing the occurrence of spurious signals.
In a received signal in the case of detection by the use of a pulse compression technique, a side lobe with respect to time called a “range side lobe” appears, as shown in FIG. 9. A small range side lobe level is a capability required for the underwater detector, for example.
Here, JP2004-177276(A) discloses that spurious signals can be prevented from occurring by making more gentle the rising up and falling edge of the envelope (hereinafter, referred to as “envelope control”) of the ultrasonic signal to be transmitted underwater.
It is also known that the range side lobe which appears in the case of underwater detection by use of the pulse compression technique can be made small by transmitting an envelope controlled ultrasonic signal.
For this reason, in order to suppress the occurrence of spurious signals and to make the range side lobe level small, an underwater detector 100 of related art performs the envelope control of the ultrasonic signal with a Pulse-Width Modulation method (PWM).
Hereinafter, the underwater detector 100 is described with reference to FIGS. 10 to 12. In the underwater detector 100, as shown in FIG. 10, a switching circuit 10 generates a reference signal VTD under a constant voltage supplied from a power source HV. At this time, a gate signal generating module 20 pulse-width modulates the reference signal VTD. Then, the reference signal VTD is applied to a transducer 30 to transmit an ultrasonic signal Pr having an envelope controlled with pulse-width modulation underwater.
Specifically, as shown in FIG. 11, the switching circuit 10 is a full bridge circuit. The switching circuit 10 is mainly composed of a FET1 to a FET4. Switching of the FETs 1 to 4 is controlled by gate signals G1 to G4 outputted from the gate signal generating module 20 (see FIG. 10), respectively. Each gate signal is a pulse-width modulated binary signal of Hi and Lo, as shown in (a) of FIG. 12. When the gate signal G1 is Hi, the FET1 is ON. When the gate signal G2 is Hi, the FET2 is ON. When the gate signal G3 is Hi, the FET3 is ON. When the gate signal G4 is Hi, the FET4 is ON.
Then, when both the FET1 and the FET4 are ON under a constant voltage outputted from the power source HV, a current in the positive direction flows to a primary side of a transformer, whereas when both the FET2 and the FET3 are ON, a current in the negative direction flows. In this way, the reference signal VTD based on a waveform of each gate signal is generated on the primary side of the transformer, as shown in (b) of FIG. 12.
The reference signal VTD generated as the above is subjected to a voltage rising in the transformer and applied to the transducer 30 such that the transducer 30 is driven. As a result, the ultrasonic signal Pr envelope controlled as shown in (c) of FIG. 12 can be transmitted underwater.
However, switching of such as the FET provided to the switching circuit 10 has a response speed limit, and thus does not respond if the gate signal does not rise for a predetermined time or more. Therefore, since a pulse width Tb of the reference signal VTD can not be set to a predetermined time or less, the envelope of the ultrasonic signal Pr, controlled by pulse-width modulation, has not been able to have the controlled width made sufficiently large.
The controlled width of the envelope is largest when the pulse width Tb of the reference signal is modulated from a response limit time (referred to as “min”) of the FET and the like to a time which is the same as the pulse interval Ta (referred to as “max”) as shown in (b) of FIG. 12. However, because even the controlled width of the envelope at this time is not sufficient, spurious signals have occurred. In addition, the range side lobe level has not been able to be made small in case of using pulse compression.